On faint companions in the close environment of star-forming dwarf galaxies

K G Noeske,K J Fricke,J M Vílchez,J Iglesias-Páramo,P Papaderos

doi:10.1051/0004-6361:20010446

Abstract

We have searched for companion galaxies in the close environment of 98 star-forming dwarf galaxies (SFDGs) from field and low density environments, using the NASA Extragalactic Database. Most of the companions are dwarf galaxies which due to observational selection effects were previously disregarded in environmental studies of SFDGs. A subsample at low redshift, km s-1, was chosen to partially eliminate the observational bias against distant dwarf companions. We find companion candidates for approximately 30% of the objects within a projected linear separation kpc and a redshift difference km s-1. The limited completeness of the available data sets, together with the non-negligible frequency of Hi clouds in the vicinity of SFDGs indicated by recent radio surveys, suggest that a considerably larger fraction of these galaxies may be accompanied by low-mass systems. This casts doubt on the hypothesis that the majority of them can be considered truly isolated. The velocity differences between companion candidates and sample SFDGs amount typically to 250 km s-1, and show a rising distribution towards lower . This is similarly found for dwarf satellites of spiral galaxies, suggesting a physical association between the companion candidates and the sample SFDGs. SFDGs with a close companion do not show significant differences in their Hβ equivalent widths and colours as compared to isolated ones. However, the available data do not allow us to rule out that interactions with close dwarf companions can influence the star formation activity in SFDGs.

Highlights

Dwarf galaxies are considered important contributors to the baryonic mass of the Universe and to the star formation rate (SFR) density at higher redshifts
We have searched the close environment of a sample of 98 star–forming dwarf galaxies (SFDGs) in field and low– density environments, taken from different catalogues, for possible companion galaxies
To supplement previous work, which had mostly dealt with luminous companions, a subsample of nearby SFDGs was chosen to moderate the effect of observational bias against low luminosity companions

Summary

Introduction

Dwarf galaxies are considered important contributors to the baryonic mass of the Universe and to the star formation rate (SFR) density at higher redshifts (see. e.g. Guzman et al 1998). Dwarf galaxies are considered important contributors to the baryonic mass of the Universe and to the star formation rate (SFR) density at higher redshifts Some of them may even be similar to low–mass building blocks of normal galaxies in a bottom–up cosmological model. A better understanding of the processes that drive their evolution is central to cosmological studies, and to the understanding of galaxy formation and evolution. Starbursts, i.e. brief episodes of strongly enhanced star formation (SF) activity, are thought to occur frequently during the lifetime of a gas–rich dwarf galaxy. Self–Propagating Star Formation (Gerola et al 1980; see Thuan 1991) or a cyclic process of gas infall onto and expulsion from an older stellar host (e.g. Davies & Phillips 1988; Papaderos et al 1996)

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Conclusion